The present invention relates to a damping characteristics control apparatus for a vehicle suspension, and particularly relates to a damping characteristics control apparatus for controlling damping characteristics of a shock absorber mounted between a sprung member and an unsprung member for each wheel of a vehicle.
With respect to the damping characteristics control apparatus, it is known heretofore that the damping characteristics of the shock absorber is controlled according to a control law of a skyhook damper, to provide C(absorber) for a damping coefficient of the shock absorber as follows;C(absorber)=C(skyhook)*[z′/(y′−z′)]wherein “C(skyhook)” is a skyhook damping coefficient, “z′” is velocity of a sprung member and “y′” is velocity of an unsprung member, respectively, in a moving direction of the shock absorber, which is the differentiated value of a displacement “z” and the differentiated value of a displacement “y”, respectively, and “(y′−z′)” is vertically relative velocity between the sprung member and the unsprung member.
The skyhook damping coefficient C(skyhook) should be of a value which is set to be relatively low for an input with its high frequency vibration component from a normal road surface prevailing, and a value which is set to be relatively high for an input with its low frequency vibration component from an irregular road surface prevailing. For example, if the skyhook damping coefficient C(skyhook) was set to be of the relatively low value, a ride comfort could be obtained against the high frequency component of the road surface, whereas, vibration of the vehicle body could not be damped appropriately on the irregular road having the low frequency component. On the contrary, if the skyhook damping coefficient C(skyhook) was set to be of the relatively high value, it could be appropriate for the irregularity, but the ride comfort on the rough road would be deteriorated. In order to solve these problems, it is proposed to change the skyhook damping coefficient C(skyhook) into a high value or a low value in response to vibration of the vehicle body, in Japanese Patent Laid-open Publication No. H05-294122, which corresponds to U.S. Pat. No. 5,324,069. That is, it is proposed to detect acceleration of a mass body (sprung member) for each wheel, and obtain signals by passing the acceleration through a low-pass filter and a high-pass filter, on the basis of which it is determined whether the vibration of the mass body has been resulted from the high frequency component or the low frequency component. Then, if the low frequency component prevails, the value of the damping coefficient C(skyhook) will be increased.
According to other means for solving that problem, in Japanese Patent Laid-open Publication No. H06-156036, it is proposed to achieve the skyhook control by changing modes for damping force in response to positive sign or negative sign of the velocity (z′) of the sprung member. Furthermore, Japanese Patent No. 2911368 discloses an equation of motion for a vehicle following a lead vehicle, based on its back and forth motion which will fit a physiology of human sensations.
Although the skyhook damping coefficient C(skyhook) is provided to be of a constant value, if the sign of z′*(y′−z′) is negative, it is required to change the skyhook damping coefficient C(skyhook) into the one of a very low value Cmini. With this low value Cmini being employed, the motion of the unsprung member will be damped. Depending on the value Cmini to be set, the changing motion will be repeated often without the motion of the unsprung member being restrained, to cause repetition of the low value and high value, which might deteriorate the ride comfort. Therefore, according to this control law, in the case where its sign of z′*(y′−z′) is negative, which corresponds to a half of the motion conditions, it will be excluded from the control, to be changed into the low value Cmini. As the control itself does not fulfill the half of the motion conditions, and the skyhook damping coefficient C(skyhook) is a fixed value even in the case where the control is effective, the equation of the control as it is could not meet every road conditions in a general state employing it.
According to the aforementioned skyhook control law, therefore, if the sign of z′*(y′−z′) is negative, the actual damping coefficient of the shock absorber C(absorber) will be of a negative value. In order to avoid this, the value employed in that period has been changed into the low value Cmini without performing the control, or a control law for changing the skyhook damping coefficient C(skyhook) depending on the frequency component of the sprung acceleration, has been added. However, only by means of that skyhook control law, the vibration of the sprung member will not be damped appropriately. This is resulted from the fact that the control shall not be performed in principle, in the case where the sign of z′*(y′−z′) is negative, and that the value of the skyhook damping coefficient C(skyhook) will have to be changed, against the vibration of the sprung member caused by a rapid change of the control conditions due to the former fact. Furthermore, as the value of the skyhook damping coefficient C(skyhook) has been set to be the fixed damping coefficient, the value of the skyhook damping coefficient C(skyhook) has to be set appropriately.
However, even if the value of the skyhook damping coefficient C(skyhook) has been set appropriately, any appropriate shock absorber damping coefficient C(absorber) can not be obtained on the basis of the aforementioned control law of C(absorber)=C(skyhook)*[z′/(y′−z′)], as long as it is the fixed value. Therefore, by changing the value of the skyhook damping coefficient C(skyhook) and Cmini depending on the road surface conditions, or providing two fixed damping characteristics for the specific characteristics of the skyhook control, to change two control modes depending on the negative or positive sign of the sprung acceleration, the skyhook characteristics has been realized artificially, with the fixed damping force characteristics, to result in a control for changing damping force characteristic modes, which is substantially different from the skyhook control law.
Also, in order to realize the skyhook control, complicated additional control means will be required. However, even if the skyhook control was completely realized, the vibration of the vehicle body suspended through the skyhook by the fixed shock absorber would be a mechanical free vibration of a spring mass system, and would hardly become a vibration based on the human physiology. In order to solve this problem, it is desired to restrain the vibration of the sprung member by means of only a single control law, without performing any change of damping force or damping force mode, which is different from the skyhook control law requiring the changeover.